Opting for shell type boilers over coil type boilers leads to availability of good quality steam and reduces the process steam requirements.
Sometimes either to meet low steam demands or to minimize the initial investment, plant opts to have a coil type boiler over a shell type boiler as the source of steam generation. However typically shell type boilers have 20% to 30% less running cost as compared to coil type boilers. One of the factors that contribute to the high running cost is the low dryness fraction of steam generated in a coil type boiler.
The coil type boiler is single pass boilers and as a result, the water needs to be converted to steam only in a single pass. This is against shell type boilers which are generally 3 pass. As a result 3 times the boiler water comes in contact with the hot gases ensuring a higher dryness fraction.
The dryness fraction represents the percentage of steam that is dry and the percentage that is moisture in a given volume. A low dryness fraction indicated steam contains more hot water and less steam. We know that in indirect heat exchange with steam, the maximum heat content is released via latent heat which is given off when steam condenses. Thus with the same steam flow rates, the energy output of dry steam will be more than wet steam. Therefore with wet steam, the process steam requirement shoots up. Additionally wet steam also reduces the life of the steam system.
Opting for shell type boilers over coil type boilers ensures the plant receives good quality steam at reduced fuel consumption.
Consider the process requirement of 2,00,000 Kal/hr,
Thus with a coil type boiler 25% more steam will be required over a shell type boiler.